The three specific aims of the proposal are as follows: 1) To determine if Dlar and Sdc protein interact in vitro: purified recombinant proteins will be produced and tested for interactions with immunoprecipitation techniques. Deletion mutants of Dlar will also be constructed to determine the domains of Dlar that interact with Sdc. 2) To determine if Dlar and Sdc interact genetically: Drosophila mutant for Dlar or Sdc will be crossed into mutant Dlar and Sdc backgrounds to determine if CNS motor axon phenotypes seen with one mutant gene are increased by removing the other gene of interest. Such a phenotypic enhancement would indicate there is some sort of Dlar-Sdc interaction in vivo. 3) To determine if Sdc acting as a ligand or co-receptor for Dlar in vivo: Sdc will be overexpressed in muscle cells or in neurons in Drosophila and motor axon guidance phenotypes examined. If Sdc overexpression in muscle cells causes a phenotype, this suggests that Sdc is acting as a ligand for Dlar. If Sdc overexpression in neurons causes a phenotype, this suggests a co-receptor role for Sdc.